High pressure discharge lamps of compact size provide a high luminous flux, and are analogous to point light sources, and light distribution control thereof is easy. Therefore, they have been widely used as an alternative to incandescent lamps or halogen lamps in recent years.
For lighting this high pressure discharge lamp, there is a discharge lamp lighting device which switches a DC voltage on and off at high frequency and operates the high pressure discharge lamp with a low frequency square wave output via an inductor and a capacitor. When a high frequency power is used to light the high pressure discharge lamp, arc discharge may become unstable because of acoustic resonance, and the lamp may flicker or go out. However, lighting by the low frequency square wave offers a stable and continuous arc discharge.
FIG. 8 shows a circuit configuration of a conventional discharge lamp lighting device. An AC voltage output from a commercial power source 1 is rectified by a rectifier 2, smoothed by a step-up chopper circuit 3, and generated as a DC power source E by a capacitor C1. Then, the power source voltage VE of the DC power source E is converted by a DC/DC converter 4 which controls lamp current or lamp power necessary to stably light a high pressure discharge lamp La. The output thus controlled to a desired value is converted into a low frequency square wave output by a DC/AC inverter 6, and then is supplied to the high pressure discharge lamp La. For start-up of the high pressure discharge lamp La, it is necessary to induce a dielectric breakdown by a high voltage. Because of this, a high voltage pulse is generated in a starting pulse generating circuit 7 to start-up the high pressure discharge lamp La. A high voltage pulse of several to tens of kV is necessary to start-up the high pressure discharge lamp La.
The start-up of a high pressure discharge lamp is characterized by a dielectric breakdown of the high pressure discharge lamp La and its subsequent transition from a glow discharge to an arc discharge, and it is essential to control energy to be supplied under optimum conditions for individual operating states and to maintain a stable lighting state from the start-up. To this end, it is required to detect a lamp voltage, and a desired control can be realized with the help of a lighting state decision circuit (comparator CP1) which determines whether the high pressure discharge lamp La is in a lighted state or in an unlighted state, and an operating state switching control circuit (microcomputer 8) which, depending on a predetermined operation phase, switches and controls the operating state of at least one of the DC/DC converter 4, the DC/AC inverter 6, and the starting pulse generator circuit 7.
The following description will be made under the premise that microcomputer control is suitable for the control accompanied with the state transition mentioned above. The state transition of the discharge lamp La after power is applied thereto and the control required in each state will now be explained with reference to FIG. 9.
First, let “start-up phase” be defined as a state where, after power is applied, a starting pulse is outputted to incur the breakdown of the discharge lamp, and thus the glow discharge is initiated. Then, the control required in the start-up phase involves performing “the output of a starting pulse” by the starting pulse generation circuit 7, “the control of output current for the glow discharge” by the DC/DC converter 4, and “the warm-up of the lamp electrode and the low-frequency control for avoiding the lamp from going out” by the DC/AC inverter 6.
An example of the “glow discharge output current control” is shown in FIGS. 10A and 10B. When a constant current is applied to the lamp, the lamp electrode can be warmed up. For the glow discharge output current control, therefore, the constant current is applied to the lamp regardless of the lamp voltage as shown in FIG. 10A so that a lamp power is increased and decreased in proportion to the lamp voltage as shown in FIG. 10B. The lamp voltage Vg during the glow discharge is in a range from 200 V to 300 V, and the voltage increases and decreases very unstably.
Next, when a transition from a glow discharge to an arc discharge takes place, the lamp voltage instantaneously drops as shown in FIG. 9. The lamp voltage Vs at this time ranges from about 20 V to 30 V. Let “stable lighting phase” be defines as a state where the lamp voltage is stabilized towards a rated voltage Vr, following the transition to the arc discharge and the lamp is stably lighted. Then, the control required therein involves performing “the stop of the starting pulse” by the starting pulse generating circuit 7, “the control of output current for the arc discharge” by the DC/DC converter 4, and “the low-frequency control for avoiding acoustic resonance phenomena” by the DC/AC inverter 6.
An example of “the control of output current for the arc discharge” is shown in FIGS. 11A and 11B. For the control of output current for the arc discharge, a lamp current is controlled depending on the lamp voltage such that it is controlled to be kept constant particularly around a rated lamp voltage Vr, wherein the rated lamp voltage Vr is generally about 100 V.
Lastly, in the stable lighting state, when a power source voltage of the DC power source E is lowered by interruption of the power source 1, the discharge lamp is liable to be distinguished. To prevent the drop of the power source voltage as much as possible, the output power may be set lower than the rated power. In general, the output power may be about 50% of the rated power. This state may be defined as “power interrupt off phase”. Then, the control required therein involves performing “the stop of the starting pulse” by the starting pulse generation circuit 7 as in the stable lighting state, “50% output power control” by the DC/DC converter 4, and “the lower-frequency control for avoiding the lamp from being extinguished” by the DC/AC inverter 6.
The control described above is illustrated in the flow chart shown in FIG. 12. After the power is applied, a threshold voltage Vth for lighting decision is set as a predetermined value Vth1 that is lower than the power source voltage of the DC power source E and higher than the rated lamp voltage Vr at the stable lighting state (step S10). The control of the lamp La is performed based on the threshold voltage Vth=Vth1 regardless of the operating state.
In the lighting deciding step S11, the lamp voltage Vla is compared with the threshold voltage Vth. If Vth>Vla (Yes in step S11), the lamp La is regarded to be in a lighted state, so the control for stable lighting is carried out in step S12. If Vth>Vla is not satisfied (No in step S11), the lamp La is regarded to be in an unlighted state, so the control for start-up is carried out in step S13 and the step goes back to step S11. In the control for stable lighting, a detected power source voltage Vb is compared with a reference voltage Vref in step S14. If Vref>Vb (yes in step S14), it is regarded as a power interrupt state, so the control for power interrupt (dimming) is carried out in step S15 and the step goes back to step S11. If Vref>Vb is not satisfied in step S14, it is not regarded as a power interrupt state, so the control for stable lighting is continued (step S16) and the step goes back to step S11.
A technique of switching and controlling the operating state of a lighting device depending on the state of a discharge lamp, e.g., a lamp voltage, as explained above is disclosed in, e.g., Japanese Patent Laid-open Application Nos. H09-069395, H07-106071 and 2007-257989.
Disclosed in H09-069395 is a technique that changes the output characteristics of the DC/DC converter depending on the voltage of the lamp at its start-up stage and in a final stage of life span, though it is not directly responding to a lamp voltage change during the start-up. Disclosed in H07-106071 is a technique that detects a lamp voltage to control the operation/stop of the start-up pulse generating circuit, based on the detected lamp voltage. Disclosed in 2007-257989 is a technique that allows the output power to be lower than the rated output in a power interrupt state.
In these conventional techniques, a threshold for deciding whether the discharge lamp is in a lighted state or not is set as a single predetermined value Vth1 that is designated by referencing only for the “stable lighting state”, wherein the threshold Vth1 is typically set higher than the rated lamp voltage Vr during the stable lighting state and lower than the power source voltage VE of the DC power source E, i.e., Vr<Vth1<VE. The rated lamp voltage Vr during the stable lighting state is in the range from about 100 V to 150 V provided that the last stage of lamp life span is also taken into consideration, and the power source voltage VE of the DC power source E is in the range from about 300 V to 450 V when conditions such as AC input voltage and the like are taken into account consideration. Therefore, Vth1 is in the range of 150 V<Vth1<300V, and, typically, Vth1 is designed in the range from about 200 V to 250 V. This value is an optimum threshold in the “stable lighting phase”.
One of reasons not considering the threshold voltages for the cases such as “start-up phase” and “power interrupt phase” defined earlier is because the “start-up phase” lasts only for a very short period of time, so that the control during that short period of time may not be considered as of great importance. Also, the “power interrupt phase” may not also be regarded as of great importance because the circuit normally turns to its stop direction after that phase.
However, when a lamp in the final stage of its life span or a lamp of poor start-up performance is combined with such a discharge lamp lighting device described above, the “start-up phase” may last for a long time, and the state of the discharge lamp may not be correctly recognized during that time. Therefore, it may not be properly controlled to perform an originally expected operation so that the start-up failure may occur or stable lighting may not be possible. Moreover, the “power interrupt phase” also has to deal with instantaneous power failure including a case where power is temporarily turned off for a very short period of time and then turned on immediately thereafter. In such a case, it is difficult for the conventional lighting device to correctly recognize the state of the discharge lamp, so that undesirable control other than the expected operation may be made due to incorrect recognition.